Equilibrium geometries and binding energy scaling relationships for aromatic excimers and exciplexes: A TDDFT and NEVPT2 study

Gas phase assocn. of polyarom. hydrocarbons (PAHs) is thought to play a key role in processes ranging from soot formation to cosmic dust growth. While small PAHs such as benzene and pyrene form relatively weakly bound van der Waals complexes in the ground state, significantly stronger binding has been obsd. in the first singlet excited state. Time-dependent d. functional theory (TDDFT) has proven an accurate and efficient method of calcg. excited state energies. However, the importance of static correlation in PAH systems means that multireference methods are often required for qual. accurate descriptions of excited states. In this work, benchmark binding energies are computed for the benzene excimer using a range of TDDFT hybrid and double hybrid functionals. Results are compared against multireference complete active space SCF (CASSCF) results with second-order n-electron valence state perturbation theory (NEVPT2) correction. Scaling relationships between dimer carbon atoms and binding energy are established using both TDDFT and NEVPT2 methods, allowing estn. of binding energy for large PAH excimers, for which multireference calcns. are not feasible. Complete potential energy surfaces are constructed for the benzene-naphthalene and the naphthalene-anthracene heterodimeric exciplexes at the double hybrid TDDFT level. The results allow a direct comparison of binding energy scaling and equil. geometry for homodimeric and heterodimeric complexes. They also provide a starting point for the statistical mech. anal. required for a complete understanding of PAH binding thermodn. and kinetics